Certain chemical and biological contaminants can be harmful or dangerous at very low levels (low parts per billion (ppb) for chemicals and 1-100 cts/ml for biological agents) and, therefore, a rapid and sensitive detection of such toxins and pollutants in air and drinking water, followed by rapid remediation, is critically important. However, many biological and chemical materials such as explosives, protein biotoxins and some microbial materials cannot be detected by current methods such as binding affinity or fluorescent assays.
On the other hand, Raman spectroscopy holds potential for a wide variety of high resolution sensing of molecular compounds in air and water for environmental protection applications. This technique measures the photon energy of scattering monochromatic light reflected when a laser illuminates target molecules. The measured shifts in photon energy provide information for molecular identification. For example, many molecules/materials of interest exhibit unique Raman spectra that can be used as fingerprints for direct molecular/material identification[1, 2].
Raman spectroscopy, however, suffers from its very weak signal strength, especially for portable Raman spectroscopy systems. In fact, the Raman scattering cross section of molecules is about 10-12 orders of magnitude smaller than the fluorescent scattering cross section of laser dyes. However, since the discovery of significant enhancements of Raman scattering using metallic surfaces or nanostructures, surface enhanced Raman spectroscopy (SERS) has been explored in different fields. Importantly, recent experimental observations have shown that in certain situations, SERS enhancements can improve signals strength by as much as 1014˜1015 times, elevating SERS to single molecule sensing levels[3-7]. This great potential of SERS can best be realized in conjunction with new plasmonic nanostructures that can not only provide extraordinary local field enhancements, but can also be fabricated via a controllable, reproducible, large scale process.
Studies have shown that large field enhancements are usually localized at the nano-gaps of nano-antennas and the enhancements improve dramatically when the gap size decreases below 5 nm[5, 8]. These studies are based on single molecule SERS experiments that typically use aggregates of colloidal nanoparticles where the “hot-spots” of enhanced local fields are obtained only by chance and are not controllable[3-7]. The particular challenge to achieving repeatable and controllable SERS active substrates originates from the difficulty of fabricating large-scale arrays of nano-gaps in a controllable and repeatable manner. The existing approaches employ MEMS fabrication processes that are extremely expensive and make these approaches non-cost-effective.
Recent studies in plasmonics, however, have led to a better understanding of surface plasmon resonance and local field enhancements that hold promise for novel designs and large-scale fabrication of single molecule SERS active substrates. For example, a variety of designs of optical nano-antennas have been proposed and demonstrated. Particularly, by varying the geometric parameters of the plasmonic nano-antennas, local field enhancements and plasmon resonance can be fine tuned[9-11]. Moreover, by confining molecules within the nano-gap between two metal electrodes, SERS and molecular electronic measurements are combined[12, 13]. Using these redundant sensing mechanisms, false positive sensor readings can essentially be eliminated.
RELATED WORK BY OTHERS: Professor Xiang Zhang's group[11] at the University of California, Berkeley developed plasmon resonance of Au/SiO2 multilayered nanodisks that exhibit several distinctive properties including significantly enhanced plasmon resonances and tunable resonance wavelengths which are tailored to desired values by simply varying dielectric layer thickness while the particle diameter is kept constant. This approach leads to higher scattering intensity and more “hot spots,” or regions of strong field enhancement. The multilayered nanodisks were prepared on quartz substrates by electron beam lithography, and electron beam evaporation followed by the standard lift-off process.
Hatice Altug's group at Boston University[17] demonstrated significantly longer plasmon lifetimes and stronger near-field enhancements by embedding the nano-antenna arrays into the substrate. This approach offers a more homogeneous dielectric background allowing stronger diffractive couplings among plasmonic particles leading to strong suppression of the radiative damping and is based on single layer e-beam lithography, reactive ion etching (RIE) and a following lift-off process.
BjÖrn M. Reinhard, et al.[18] fabricated nanoparticle cluster arrays with total lateral dimensions of up to 25.4 μm×25.4 μm on top of a 10 nm thin gold film using template-guided self-assembly. This approach provides precise control of the structural parameters in the arrays, allowing a systematic variation of the average number of nanoparticles in the clusters (n) and the edge-to-edge separation (Λ) between 1<n<20 and 50 nm<Λ<1000 nm, respectively. Electron beam lithography and lift-off process are used to define binding sites on which the dielectrically coated nanoparticles self-assemble.
The above approaches for fabrication of plasmonic nano-antenna arrays require very expensive processes such as electron beam lithography, and electron beam evaporation and lift-off processes. These processes are limited to fabricating micron scale arrays that are not scalable to the desired commercial applications of the preferred embodiment of the present invention.
Professor P. Van Duyne's group at Northwestern University[16] developed an inexpensive, simple to implement, inherently parallel, high throughput, nanofabrication technique capable of producing an unexpectedly large variety of nanoparticle structures and well-ordered 2D nanoparticle arrays. The process involves drop coating polymer nanospheres onto a substrate and then allowing the nanospheres to self-assemble into a close-packed hexagonal array. The array is then used as a mask for the creation of several different SERS active substrates. However, this approach suffers from non-scalability and is uncontrollable because the nanoparticle array is based on evaporation of a solvent from a single droplet resulting in an uncontrollable close-packed hexagonal structure.